This invention relates to a brake cylinder employed such as in a drum brake device for a motor vehicle and more particularly relates to a brake cylinder including an automatic shoe clearance adjustment mechanism.
A conventional brake cylinder device including an automatic shoe clearance adjustment mechanism as disclosed in the Japanese Provisional Patent Publication No. 9-229115 is composed of a piston slidably installed in a cylinder bore formed in a cylinder body; an adjustment bolt, one side of which has a first external thread stem making a screw engagement with an internal thread formed in the piston preventing the rotation between the two threaded portions by an axial thrust while the other side of which has a clutch surface making a clutch engagement with a first clutch surface formed inside of the cylinder bore; a clutch ring, an axial portion of which has an internal thread making a screw engagement with a second external thread stem formed on the other side of the adjustment bolt with a backlash between the two allowing a rotation of the two threaded portions by an axial thrust and a periphery of which having a clutch surface making a clutch engagement with a second clutch surface inside of the cylinder bore; and a clutch spring energizing the clutch ring in the direction to make the clutch engagement with the second clutch surface inside of the cylinder bore, wherein a straight brake fluid feeding groove is formed on a periphery at one side of the adjustment bolt.
Hereafter, xe2x80x9cscrew engagement preventing the rotation between the two threaded portions by an axial thrustxe2x80x9d may be called xe2x80x9cirreversibly screw engagementxe2x80x9d, while xe2x80x9cscrew engagement allowing a rotation of the two threaded portions by an axial thrustxe2x80x9d may be called xe2x80x9creversibly screw engagementxe2x80x9d.
A brake fluid feeding groove is explained next with reference to FIG. 8. The brake fluid feeding groove c on an adjustment bolt a is designed to be rectilinearly parallel in an axial direction of the adjustment bolt and to entirely cover a periphery of an external thread stem b on the adjustment bolt making a screw engagement with a piston d so as not to rotate the two threaded portions by an axial thrust. The brake fluid feeding groove c and the external thread stem b have been machined by either one of the following two processes. Firstly, the brake fluid feeding groove is machined by a milling machine, since then the external thread is machined by a lathe. Secondly, the external thread is machined by a lathe or a rolling machine, since then the brake fluid feeding groove is machined by the milling machine.
The adjustment bolt as a component of the aforementioned brake cylinder has the following drawbacks. If the brake fluid feeding groove c is machined by the milling machine prior to machining of the external thread, although there is no need to worry about a burr formed on a threaded portion, thread-cutting an external thread on the external thread stem b must be machined by another type of machine (lathe). Transporting and replacing of the work require more time and manpower, resulting in an increasing price of the adjustment bolt a.
If the brake fluid feeding groove c is machined by the milling machine on the heels of processing the external thread, there is some possibility of leaving a burr on the threaded portion even with a great care of the machining condition and the machining tools.
In order to avoid leaving any burr, the external thread is preferably processed prior to the brake fluid feeding groove c. However, since the brake fluid feeding groove c is designed to be rectilinearly parallel in an axial direction of the adjustment bolt a, a plastic deform processing by the rolling machine suitable for a mass production of the external thread becomes unstable in its rotation when rolling a half-finished work of the adjustment bolt a, thereby requiring a larger manufacturing tolerance. Accordingly, that kind of the processing is not appropriate for an adjustment bolt which needs to keep a deviation of backlash bellow.
A back chamber is formed between the piston d and a top of the adjustment bolt a. Brake fluid flows along the brake fluid feeding groove c and reaches to a bottom of the back chamber and rebounds toward a through duct formed at the center of the adjustment bolt a.
This invention was made to remove the aforementioned drawbacks, and an object of this invention is to provide a brake cylinder device with an excellent processability and an excellent air bleeding.
A cylinder device of this invention has an automatic shoe clearance adjustment mechanism, comprising a piston slidably fit in a cylinder hole of a cylinder body. An adjustment bolt is provided, one end of which has an external thread irreversibly screwed into an internal thread formed in the piston while the other end of which has a clutch surface making a clutch engagement with a bottom surface of the cylinder hole facing the piston. A drive ring is provided having an internal thread formed at its axle portion reversibly fitting over an external thread axle formed at the other side of the adjustment bolt with a slight gap inbetween and having a peripheral clutch surface making a clutch engagement with the bottom surface of the cylinder hole; and a drive ring spring energizing the drive ring in the direction to make the clutch engagement, wherein a spiral brake fluid feeding feed groove is formed on and entirely covering a peripheral surface of the irreversible external thread axle at one side of the adjustment bolt. Accordingly, this invention provides an advantage in which the cost for manufacturing the adjustment bolt may be reduced and the effectiveness of the air bleeding may be improved.
According to one aspect of the invention, the brake fluid feeding groove on the adjustment bolt is formed by the same rolling or turning process of forming an external thread on the external thread axle. This invention provides another advantage in which processing to form the external thread stem on the adjustment bolt and the brake fluid feeding groove may be performed by the same processing machine.
Further, this invention provides a brake cylinder which is characterized in that the brake cylinder is a two-side open type being partitioned into two sections to compose a pair of right and left cylinder bores and has the automatic shoe clearance adjustment mechanism at one of the cylinder bore. Accordingly, this provides the same operation and the advantages as described above.
Further, this invention provides a brake cylinder which is characterized in that the brake cylinder is a one-side open type being partitioned into two sections to compose a cylinder bore and has the automatic shoe clearance adjustment mechanism therein. Accordingly, this provides the same operation and the advantages as described above.
Further, this invention provides a brake cylinder which is characterized in that the brake cylinder is a two-side open type being partitioned into two sections to compose a pair of right and left cylinder bores and has the automatic shoe clearance adjustment mechanism symmetrically provided at each cylinder bore.